K9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. He served as the template for all future versions of K9.
When the Fourth Doctor and Leela arrived at the Foundation, they took a liking to K9 and he was eventually given to them by the Professor as a thank-you for defeating The Swarm. From then on, the Doctor saw the robot as a good friend. After a number of adventures with Leela and the Doctor, Mark I decided to stay on Gallifrey with his "mistress" Leela.
K9 was often left behind in the TARDIS and so he saw journeying out as a treat. He stated that to wish was to have emotion and so he wished for nothing. Despite this he regularly showed emotion, even sulking when shouted at. The Doctor described him as overbearing, arrogant and insufferable, after K9 told him there was only a 1% chance of the Doctor having a better explanation for something than him.
The mechanics of K9 Mark I included rotating ear-probes, telescopic 'eye' probe, extendable nose, flashing lights on the top and the 'eye' panel, waggling tail antennae, and ticker-tape tongue. The head could also move up and down. The shell was painted in gold/grey metallic. Around K9's neck was a tartan collar, from which hung a silver disk.
